Revision control
++ <?xml version="1.0" ?>
++ <scxml xmlns="http://www.w3.org/2005/07/scxml" version="1.0">
++ <state id="wrapper">
++ <transition event="OP.INSERT">
++ |if cond="_event.data.operator == 'OPER.PLUS'">
++ <assign location="long_expr" expr="long_expr+'+'" />
++ <elseif cond="_event.data.operator=='OPER.MINUS'" />
++ <assign location="long_expr" expr="long_expr+'-'" />
++ </if>
++ </transition>
++ </state>
++ </scxml>